Pneumatic-Assisted Control Valves

Pneumatic (air) assisted control valves are used to control
the flow of water and liquid food products, petroleum products, and other
chemicals through large piping. These types of valves are opened and closed by an electrical signal that's converted to air pressure. Pneumatic-operated
valves became popular prior to the advent of electronic and solid-state
components being used in industrial electronics. Pneumatic systems could
provide analog control to open and close very large valves smoothly.
The pneumatic-controlled valves allowed the valve opening to be anywhere
between full closed and full open. The signals for these systems were
transmitted over long distances throughout a factory or process plant
by transmitting air through plastic tubing or copper tubing. Pneumatic
technology has proven so dependable it has been incorporated with modern
electronic systems and controls. Hence, new technology today can use
an electronic or microprocessor controller and use an electronic-to-pneumatic
converter that changes a milliamp signal to a pneumatic signal. The electrical
signal is proportional 4-20 mA and the air pressure is proportional 3-15
psi.

These types of valves are commonly used as loads in the process industries.
They're also used in applications where explosive atmospheres exist
such as in spray painting and chemical processing.